Radio direction finder



June 4, 19.35. J. w. GRI-:1G f 2,003,932

RADIO DIRECTION FINDER Filed Dec. 27, 1932 2 Sheets-Sheet 1 Pansa Pam/enr Ava/aww):

Z2 /F HMP 24 2mn .per

INVENTo'R June 4, 1935. J. w. GREIG I n RADO DIRECTION FINDER Filed nec. 2v, 1932 2 sheets-sheet 2 h u w Q ww Patented 4,

UNITED STATES PATE NT OFI-*ice- Claims.

My invention pertains to improvements in radio `direction findingapparatusfor determining the direction of arrival of radio waves in a .horizontal plane, and is especially adaptable for 5 use as a guide to the pilot of aircraft whereby such aircraft may readily be directed in the direction of a landing field during periods of low visibility such as fogs, rains and storms, etc.

It is well known in the art of radio direction finding that a vertical loop may be employed to determine the vertical plane of arrival of radio waves, since when the plane of the loop is at right angles to the plane of arrival, themagnetic eld of the radio waves cuts both sides of the loop at the same instant and the E. F. in-

duced in one side is in exact phase opposition to the E. M. F. induced in the other side so that the resultant output E. M. F. is z ero. However, when the'plane of the loop is not at right angles tofthe plane of arrival, the E. M. F. induced in the nearer conductor of the loop will be advanced in phase over the E. M. F. induced in the farther conductor of the loop, with the result that the difference between the two E. M.' F.s .will not be zero', but will be an E. M. F. which is substantially in phase quadrature to the E. M. F.

induced in the conductors of the loop'. The mag-- nitude of this output E. M. F. will be proportional to the sine of the angle of deviation from the perpendicular position, and its phase polarity a function of whetherjthe right or left edge of the loop is nearer to the direction of arrival of the radio waves.

It is also well known in the art that the direction of arrival of the radio 'waves may 'Abe determined by a measurement ofthe magnitude and phase polarity of the output of the loop, and means have been described by previous inventors for the use of this principle.` 4For example, if a vertical antenna is placed close to the loop, an E. M. F. will be induced in this antenna which is not a function of the direction of arrival of the radio waves, and which may be used as a standard of comparison for the determination of the relative magnitude andphase polarity of the output E. M. F.. of the loop. The simplest form of measurement may be made by providing a reversing switch wherebythe output of the loop may be added to the output of the vertical antenna with either normal or,reverse poling, and comparing the magnitudesof the outputs from a radio receiver supplied by the loop and vertical antenna in combination for the condition of normal and reverse polingA of the switch. If the output of the receiver is the same for both normal and reverse vpoling, the direction ,of arrival of the radio waves is at riglrt*u angles to the loop,`since the loop has then no output E. M. F. and has no effect when combined with the output from the vertical antenna.

(ci. 25o-11) However, when the direction of arrival is to the right of the perpendicular position, the output of the loop will add to the output of the' vertical antenna with the reversing switch in the normal position and will subtract when the reversing switch is in the reverse position, (assuming such poling and suitable Vphase shift means to give this effect). Conversely, when thedirection of arrival is to the left of the perpendicular position, the output of the loop will subtract from the outlput of the vertical antenna when the reversing switch is in the normal`position, and will add when the reversing switch is in the reverse position. Therefore the relative right or left direction of arrival maybe determined by an observa tion of the position of the reversing switch which gives the greatest output from the receiver. and the 4approximate magnitude' of the deviation from the'perpendicular to the plane of the loop determined by the difference in the output of the receiver with the reversing switch in the normal position and the output with thereversing switch inthe reverse position.

An important factor in the use of the above system is the provision of means to shift the 4phase of the output of the loop to approximately coincide with the phase of the output of the vertical antenna, since if the vertical antenna is located at .the center of the loop and both loop and vertical antennas are tuned to resonance.

the output current of the lop will be in phase quadratureto the output current of the vertical antenna, and the sum of the two currents will be the same for both the normal and reverse polings of the reversing switch. Some phase shifting device must therefore be placed in either the output of the loop or antenna or both, which -produces a relative phase shift of approximatelydegrees before the combination of the output current of the loop with the output current of the vertical antenna onthe receiver input.

Various methods have been heretofore pro-v posed for an automatic operation of the reversing switch and automatically indicating the position of the switch which gives the maximum output from the receiver, in order to make the system suitable for use on aircraft or other mobile vehicles. In the most practical forms, the reversing switch or some equivalent device has been operated at a high speed of alternation' so that the alternate addition and substraction of the loop and antenna outputs have produced --reversing means, and the relativeplus or minus phase polarityindicated by a zero center meter, and this polarity employed as an indication of a modulation ofthe carrier applied to the radio.

necessary -to make the output of the loop additive or subtractive to the output from the vertical antenna is secured by means which reduces ,the efficiency ofthe' system, such as mistuning the antenna or loop circuits or introducing a re-- actance-resistance phase shifter. It is also diiilcult in such systems to secure a phase shift which remains constant at 90 degrees over a band of frequencies.

It is an object of my invention to provide a means whereby the phase shift may be accurately adjusted to 90 degrees.

-It is a further object of my invention to provide a means whereby the lphase shift may be maintained constant over a range of frequencies. Another object is to provide a reversing means, said switch comprising al1-arrangement of rectiiers'of the copper oxide, vacuum or gas tube types. y A still\further object of my inventionis to provide a means whereby the percentage modulation produced by the reversing device is con-` ,stant over a range of frequencies, and whichin combination with-an automatic gain control'will make the deection o f the indicating meter a constant function of the deviation of the direction of the inco'ming signal from the perpendiculartotheloop. A'

` The novelfeature of my invention whereby the above objects are attained is the use of a superheterodyne type of radio receiver with two input channels, two or more modulators, means in the intermediate frequency circuit for periodically reversing the loop channelwith respect to thevertical antenna channel, and means inthe' intermediate frequency circuit for shifting the phase of the current from the loop channel to antenna channel. 'I'his feature, as well as other novel features which I believe to be characteristic of my invention are set forth in the appended claims. 'I'he invention itself, however, both as to its organization and method of operation together vtages thereof, will best be vunderstood Iby reference to the followingdescription when read in connection with the accompanying drawings wherein- Figure 1 is a schematic diagram of the circuit arrangement of Vone embodiment of my direction,

finder;

Fig. 2Y shows how a Icopperoxide full wave rectifier may be employed as a reversing means; Fig. 3 is a schematic diagram of a form of modulator circuit which minimizes 'crosstaik methods of employing vacuum tubes for a reversbetween the loop and antenna channels:

Fig. 4 is a schematic diagram of valternative ing means;

' ure 1 showing the preferred form of circuit, I.

Fig. 5 isa schematic diagram of a novel method of employing push-pull modulators as a reversing means. 1 The'lament heating circuits have been omitted from these circuits as means for engaging the filaments are well known to those versed in the art."

Making reference to the accompanying drawings and rst directing special attention to Figwill describethe lvarious features of my inventiom L Loop antenna I is shown as inductively cou-` with further objects and advan the relative direction of deviation of the direction of arrival from the perpendicular to the loop.

In the systems of the prior art the phase shift pled to the tuned input circuit 3 consisting of inductance 3 and condenser 4, which is associated with the input to the rst detector or modulator tube 5. An electrostatic shield 3" is shown between' the loop winding andthe inductance 3' in-order Ato prevent electrostatic transferof energy from the loop which would cause' an error in the directional indication. 'I'he loop may be asingle turn transverse to the longitudinal axisY of the ship, or it may 'be a smaller multiturn loop capable of rotation about a vertical axis.

Vertical antenna ,2' is shown as capacitively coupled to the tuned input circuit 6 consisting of inductance 6 an'd condenser 1, which is associated with the rst de'tector or modulator tube 8. 'Ihe two condensers 4 and l are considered as of the variable type ganged together in awell known manner so that the capacities are very nearly equal at all dial settings. l In order to make the phase shift introduced in the loop channel by tuned circuit 3 equal at all dial settings to the phase shift introduced in the verticall antenna channel by tuned circuit-8, a compensating condenser 9 is connected in the loop channel to balance the capacity introduced by the antenna in tuned circuit 6, and a compensating inductance I I is introduced in the antenna channel to compensatefor the eifect of the loop on the tuning ofl circuit 3.

The advantage of maintaining a high degree of' accuracy in the phase 'shift characteristics of these circuits will be explained later in the explanation of the action of the reversing mechanism..

While the loop antenna is shown as separate from the vertical antenna in Figure l, it is not intended to limit my invention to use with any particular form of antennas. For example, the

-vertical antenna might be replaced by a midtap connection to lthe loop. Another variation would be the use of two vertical stub masts with the outputs connected in series opposition to replace the loop.

The use of the two modulators 5 and 8 constitutes the most unique feature of this invention. By thus converting the output from the loop and the output from the vertical antenna to a fixed.` and comparatively low intermediate frequency prior to combination in a common ampliner, it becomes possible toiperform the reversal operation'with high eflciency by means of a copp'er oxide rectifier or`other equivalent device. The use of a fixed frequency insures that the degree of modulationproduced by the reversal Voperation is a constant percentage of the carrier current. Furthermore, the use of a fixed frequency permits the maintenance of a con.- stant phase shift by means of a phase shifting network .hereinafter referred to.

The two modulators are shown as. vacuum tubes of the tetrode type -with the beating radio .frequency supplied to the s creen grids thru The ouput ofmodulator 5 is connected to the' reversing rectifier I6 thru a tuned impedance matching transformer I5. The reversing means shown herein may-be a bridge connected copper oxide rectifier ofthe type commonly employed Aing voltage is applied to the a. c. terminals as an instrument rectifier for alternating current measurementa- ,The action of full wave rectifier as a reversing means may be best explained by reference to Figure 2.

The intermediate frequency .input is connected to the d. c. terminals of the rectifier, vshown in .,Flgure 2A as'terminals a and c. VThis input is the reversing voltage is zero, the rectier is virquency voltagebecomes positive on the d termi- '30 tually a balanced bridge so that no energy is transferred from terminals, a-cto terminals d -b. When the low frequency voltage becomes positive on 'the b terminal, the bridge arms bc and ad become conductors so that the input leads are connected to the output leads` with reverse poling. Conversely, when the low frenal, thebridge arms dc and ab become conductors so that the input leads are connected to the voutput leads with normal poling.

Under these( conditions, as is well known to those skilled in the art, the actual output of the rectifierwill consist ofv sideband frequencies only, with the carrier suppressed. If the variation of `impedance ofthe rectifier be substantially linear with respect to the low frequency voltage applied, the sideband frequencieswill be the carrier frequency plus and minus the low frequency employed for reversing the rectifier. 'Ihe low frequency input is made high compared to the intermediate frequency input so that the rectier .is virtually a linear conductor for the intermediate frequency and therefore but little distortion is produced in the intermediate frequency.

Referring again to Figure 1, the intermediate frequency sideband. output from rectifier I6 is passed thru phase shifter I'I. The type of phase shifter shown as the preferred form is a well vknown lattice network which has the property of shifting phase without introducing attenuation. .The values of the inductances and capacities which make up this network are to be adjusted to a value which produces a phase shift of approximately degrees at the frequency of the intermediate carrier. They should also be chosen of such Value that the phase. shift is vgradual thru the 90 degree point, so. that jslightinaccuracies in tuning ofthe oscillator will.y not Aproduce an excessive error in phase shift.-

While the phase shifter shownV in Figure 1 is of the preferred form,'other forms might be employedwithout exceeding the scope of my The output of the phasev shifter is connected to -the inductance I8, which is shown as com- 'posed of. two balanced .sections connected in series aiding thru the bypass condenser I9, It is recognized that other methods of supplying the low vfrequency voltage to the rectier might be employed, such as the' arrangement shown in Figure 2. r i

' Rererrmg'nbw to'tneputput o: the antenna channel, the outputv of modulator tube l is connected to the tuned output transformer` 2h, which in turn is, connected to the inductance 2I,

thru a balancing network 22. The m1111082 of f this balancing network is to introduce a'phase change and attenuation in the antenna output channel which is equivalent to the phase change and attenuation produced in the loop channel by the reversing means It. While this network 22 is not essentialto the theoretical operation ,of my invention where an absolutely fixed intermediate frequency is assumed. it is desirable in a practical embodiment of the-invention where slight variations of the intermediate frequency may be expected due to mistuning and variations of the oscillator frequency.- 'Y

A resistance capacity network is shown in Figu re 1 as the balancing network 22, in orderto be equivalent to the impedance presented by the copper oxide rectifier employed asthe reversingy means I6. 'If other reversingmeans were employed, a corresponding change would be made in the balancing network.

Both inductance coils 2I and 'I8 ae coupled to inductance coil 22, which in connection with condenser 23 forms the tuned input circuit 23' to the common intermediate frequencyamplifier. indicated as a whole asat 24. Since coil 2l carries the intermediate frequency carrier current while coil I8 carries the sidebands produced from the intermediate carrier by the action of the reversing means I6, the addition of the two in the common tuned circuit 23'. produces a modulated carrier of intermediate frequency which is applied to the common intermediate amplifier 24.

tion of a carrier `and two sidebands may be reversed degrees by the reversalof the phase of the carrier or 'by the reversal of the phase of the sidebands, pmvidedtnar the sidebands are not effectively in phase quadrature with'the carrier. While the sidebands are of different freq uency from each other and from the carrier, they may be considered as having a definite phase relation over a period of time which is small in comparison with the length of the modulation cycle. For example, if the two sidebands both pass thru a maximum on that part of the modulation cycle that the carrier is passing thru zero, the sidebands may be said to be effectively in phase quadrature with. the carrier andA the resultant envelope will be of double the modulation frequency. However, if the sidebands pass thru a simultaneous maximum whenthe carrier is passing thru a maximum with the same polarity, the'sidebands are effectively in phase with the carrier and the resultant envelope will be of the fundamental modulation frequency. With such a phase relation, the eectof reversing either carrier or Vsideband phase will be to defer the time when the three components Areach a simultaneous maximum for one half of the modulation cycle, and the phase, of the envelope Vrepresenting the modulation frequency will be reversed.

Since the phase of the sidebands is reversed y duced by the direction of the arrival vof the radio As is well known in the radio art, the phase Y of the audio envelope produced by the combinal waves being shifted fromi the right to the left of the perpendicular to theplane of the loop, `it may be seen that the phase polarity of the audio envelope of the intermediate frequency applied to intermediate frequency amplifier 24 will be an indication of the direction of'the incoming radio waves.

Furthermore, since the, directional indication is based upon ,a measurement of the magnitude '.10 as well as the'phase of the fundamental component of the modulation produced by the reversing-device, and since the relative magnitude of the fundamental component of the modulation is dependent upon the'effective phase relation between carrier and sidebands, accuracy -of indication is dependent upon the maintenance of a constant phase relation from `the antennas thru to the point of combination at the common intermediate frequency, amplifier. It is in the provision of means for securing such' an accurate phase `relation that the invention described herein has kadvantages over previous systems of a similar nature. The circuit from the iirst intermediate frequency amplifier 24 to the audio output from transformer 25 is typical of any of the radio receiver circuits, commonly employed for4 radio.

` telephone reception and has no features which are unique to this invention. An automatic gain control is a desirable but not a necessary feature. The audio output of the receiver is shown as ever, unique to this invention, as it isl intended that this circuit be detuned from vresonance to the frequency generated by the audio oscillator connected to an audio frequency tuned circuit consisting of a condenser 26 and an inductance 2l. A balanced rectier 28 is connected in shunt to the inductance 2'|.l This rectifier may be of the copper oxide full wave type similar to that employed for the reversing means I6, reconnected .as shown in Figure 1. The balanced rectifier is continuously supplied with audio frequency current from the audio oscillator 29 thru a balanced resistance 30, preferably at a high enough energy level to operate the rectier elements on the linear part of the characteristic.

.'I'he slider on the resistance -30 is to be adjusted to give a zero center indication on themeter 3| when no audio output is impressed from the receiver. l lSince the audio output from the receiver produced by the modulating action of the reversing means I6 is synchronous with the audio output from the receiver, it will add to the audio currrent derived directly from the audio oscillator on one half of the rectifier 28, and vsubtract on the other side, thereby producing a deflection of the meter 3| whose .magnitude is proportional to the magnitude of the audio output ofthe receiver and whose sign is a xfunction of the relative phase polarity between the audio outpu from the receiver and the current derived directly from ,the audio-oscillator. Since the phase of the audio output from the receiver is a function of the phase of the modulation envelope of the intermediate frequency applied tothe intermediate amplifier 24, which has been shown to be function of the deviation of the direction of the arrival of the radio Waves from the perpendicular to the plane of the loop, the meter by an amount sufcient to bring the audio output from thereceiver in yapproximate phase 5 ,"synchronism with the current derived directly from the audio oscillator v29.' 'Ihis adjustment is made necessary `by the shift in phase introduced by the frequency selective\cir'cuits in' the intere mediate frequency amplifier.

While Figure 1 shows the modulator tubes to be of the tetrode or screen grid type, it is not intended to limit this invention to any particular form of modulator. For example, a pentode type of vacuum tube may be employed, as is` .15 shown in Figure 3. In this circuit the beating oscillator O supplies a beating frequency to the suppressor grids 32.0f modulators Mi and M2. The suppressor grids are coupled to a low impedance lpoint on the oscillator thru a block- 20 ing condenser 33 of low impedance to the beating frequency and to the intermediate frequency. This circuit-,has the advantage that there is very little reactionbetween the two channels, since the suppressor grids are shielded from the con- 25 trol grids 34 by the screen'grids 35 and'since the suppressor grids are so biased as to draw little or no current.

While this invention has been described in the preferred form as using a full Wave copper oxide 30 rectifier as the reversing means, it is not intended to limit the scope of the invention to any specific means of reversing the loop output, provided that such reversal be performed at a fixed intermediate frequency and not at the radio 35 v .ployed `as the reversing means, or a rotating coil, or a rotating condenser, 4or a polechanger of the vibrating or commutator type.

In particular, a pair of amplifying vacuum tubes might. be employed in the connection shown in Figure 4 as reversing means. This connection shows two vacuum tubes 36 of the tetrode type with the grids 31 connected in 45 parallel to the modulator output 38 and the' i plates 38' connected vin push-pull to the intermediate frequency amplier 39. The screenY grids are supplied with low frequency alternating current from an audio oscillator, 'so con- 50 nected in vphi/ase .opposition that the tubes perate alternately, thereby-reversing the phase ofh the output at the frequency of the audio' supply. Y Alternative means might be used, suchas con. nesting the grids in push pull and the plates in 55 parallel.

Another variation from the preferred form which still comes within the scope of my inven-A tion is shown in Figure 5. In this case a pair of vacuum tubes 40 is employedfor both revers- 60 ing meanszand modulator. A pentode type of tube is preferred for this connection with the screens 4| supplied by audio frequency 4I' in phase opposition so that the tubesA are energized alternately. The suppressor grids 42 are shown 65V as supplied with a heterodyne frequency from 3| may be calibrated left and right, and may be further calibrated in degrees .left and right if '-70 the carrier input to the second detector is kept at `some predetermined value.

'rneuse of a balanced rectifier to 'indicate' phase polarity in the manner described above is well known to those versed in the art. The 75 tuned audio frequency circuit 26--21 is, howthe beating frequency. oscillator. The control vgrids 43 are connected in parallel tothe radio frequency input circuit 44 and the plates 45 are connected in push pull to the intermediate frequency output 46. Still other forms of this same circuit might be employed, such as connecting the grids invpush-pull and the plates in parallel; or connecting the grids in parallel, the plates inparallel, `and the suppressor grids 7 5 in push-pull: `The same principle might be employed with either tetrode or triode tubes by a combination of the various control and modulating voltages on the same electrodes.

A still further variation which might be employed would be the .use of an audio frequency `-feed back circuit in combinationwith'the connections of Figures 4 or 5 so that the pair of tubes which act as the reversing means would also act as an audio oscillator.

In the application of my invention to aircraft it is expected that the meter shown as 3| in Figure l would be mounted on the instrument board in connection with the other navigating instruments. `It is preferred that the connections to the meter be so poled that the needle deilect to the right when the nose ofthe ship turns to the right of the direction of `the incoming radio waves, since this is an established convention used for the well known turn indicator. Then when the ship is iiying toward the station, a semi-automatic response of the pilot in keeping the needle centered will bring the ship overthe radio transmitter to which the be compensated for by fiying the plane with the needle a certain amount oir center, thus crabbing I 'the' plane ""'enough i to allow FAfor the 1--cross wind. An occasional reference to the compass courser would show when the proper drift had been allowed for.

As for example the use of a multi-element tube as a modulator which incorporates a double ldiode rectifying element to replace the external rectifier as a reversing means.

Although specific embodiments of the invention have been illustrated and described, it will bel understood that various alterations in the 1 details of construction may be made without departing from the scope of the invention, as indicated by the appended claims.

I claim:-

1.' In a radio direction finding receiver having two separate input channels with a reversing means in one channel, the provision of an electrical network in the other channel having) substantially the same input impedance and transfer impedance as the said reversing means.

2. A radio receiving apparatus for indicating the vertical vplane in which 4radio frequency signals arrive with respect to an antenna system; consisting of a loopa==vertical antenna, a loop tuning means, a vertical antenna tuning means, a frstlmodulator having an input circuit connected to said loop tuning means, a second modulator having an input circuit connected to said antenna tuning means, a radio frequency oscillator electrically coupled to both of said modulators to deliverfa frequency which beats with the incoming signal frequency to form an intermediate frequency, two separate intermediate frequency channels connected to the output circuits of the said two modulators, a reversing means` in one of said channels, a phase shifting means in the other of said channels, an audio frequency oscillator connected to operate said reversing means, an intermediate frequency amplifier having an input circuit connected to the outputs of both of said intermediate frequency channels, a detector, and means connected to said detector and to said audio oscillator for indicating the relative. phase polarity. 'of the audio output of the detector as compared with the phase of said audio oscillator. 3. A. radio receiving apparatus for indicating the `direction of arrival of radio signals, comprising a loop, an antenna, a loop tuning circuit, an antenna tuning circuit, a rst modulator supplied with radio frequency energy from the loop tuning circuit, a second modulator supplied with radio frequency energy from the antenna tuning circuit, a radio frequency oscillator supplying a radio frequency to both of said vmodulators which differs from the radio frequency in the loop and' antenna circuits by an intermediate frequency, an audio frequency oscillator, means for reversing the phase of the intermediate frequency output of one of said modulators at the frequency of the said audio oscillator, means for shifting the phase of the output of one of said modulators with respect to thephase of the output of the other `of said modulators, means for combining the sidebands of the intermediate frequency produced by the reversing means with the intermediate frequency carrier, means for amplifying the said combined intermediate frequency 'carrier and sidebands, a detector, and phase polarity indicating means connected to both said detector and the said audio oscillator. y

4. In combination, a radio direction finding receiver, vselectivelytunedi intermediate -frequency circuits in said receiver for producing al phase shift in the output of said receiver, a network receiving the output of said receiver, said network consisting of an inductance, a capacity in series with saidinductance, an indicating meter in series with said inductance, a balanced rectier shunted around said inductance and said meter, an audio oscillator coupled to said rectifier to continuously supply current thereto, said capacity and said inductance being adjusted to produce a phase shift in `the audio frequency appearing in said receiver output, which' is equal and opposite to the phase shift produced by said selectively tuned phase Shifters.

5. In a radio receiving apparatus for indicating the vertical plane in which radiosignals arrive, comprising a loop, a vertical antenna, a looptuning means, a vertical antenna tuning means, first and second modulator tubes, said tubes being multigrid` type vacuum tubes including input grids and screen grids, said input channel having substantially the same input impedance and transfer impedance as the said reversing means.

\ JOHN W. GREIG. l 

